1. Field of the Invention
The invention relates to a quaternary FSK (Frequency Shift Keying) receiver for receiving a signal to which quaternary FSK is applied, and further to a method of dealing with signals in a quaternary FSK receiver.
2. Description of the Related Art
The quaternary FSK communication system is a system wherein quaternary gray signs "00", "01", "11" and "10" correspond to four different frequencies such as f1, f2, f3 and f4. A quaternary FSK receiver used for quaternary FSK communication receives signals to which quaternary FSK is applied, and judges which frequency a frequency of the received signals correspond to among the four frequencies f1, f2, f3 and f4, to thereby demodulate the received signals.
In the above-mentioned signal sign judging in a quaternary FSK receiver, a frequency of the received signals is converted into a voltage by means of a frequency-voltage converter, and the thus obtained voltage is compared to three reference values, that is, first to third reference values, to thereby judge a sign of a signal. Herein, the first reference value is set greater than the second reference value, and the second reference value is set greater than the third reference value.
In the process of signal sign judging, for instance, if the obtained voltage is equal to or greater than the first reference value, a sign of the received signal is determined to be "10", if the obtained voltage is smaller than the first reference value and equal to or greater than the second reference value, a sign of the received signal is determined to be "11", if the obtained voltage is smaller than the second reference value and equal to or greater than the third reference value, a sign of the received signal is determined to be "01", and if the obtained voltage is smaller than the third reference value, a sign of the received signal is determined to be "00".
In a conventional quaternary FSK receiver, the above-mentioned first to third reference values to be used as a reference for judging a sign of a received signal are fixed values. However, if the first to third reference values are fixed, there is caused a problem that an error occurs in signal sign judging, if a central voltage in a detecting signal due to fluctuation in a frequency of transmitted signals, variation in a temperature of a quaternary FSK receiver, fluctuation in a power source voltage, and so on.
In order to solve such a problem, Japanese Unexamined Patent Publication No. 1-164151 has suggested a method of judging a sign of a received signal, in which a voltage level of respective signs is measured by means of a capacitor, and voltages used for judgement, established based on the thus measured voltage levels, are used as reference values.
In a quaternary FSK receiver having been disclosed in Japanese Unexamined Patent Publication No. 7-154434 (Japanese Patent No. 2639326), data is sampled while a preamble signal is being detected, and the thus sampled data is converted into digital signals to thereby detect a gap between an average value of received data and a central value of reference values, which corresponds to the second reference in the above-mentioned case. Then, a sign of a received signal is judged using the average value of received data as the central value of reference values.
However, in the method having been suggested in Japanese Unexamined Patent Publication No. 1-164151, a voltage accumulated in a capacitor is used as a reference value, and the voltage or the reference voltage is varied only at a specific timing. In the quaternary FSK receiver having been disclosed in Japanese Unexamined Patent Publication No. 7-154434, the reference voltage is altered only while a preamble signal is being detected.
Hence, if a reference value is set in wrong, it would take much time to change the wrongly set reference value into a correct reference value, and the wrongly set reference value could not follow small fluctuation in a demodulated signal.
In addition, the conventional quaternary FSK receiver carries out only to harmonize an average value of received data to the second reference. The first and third reference values are set equal to fixed values which are obtained on the assumption that a demodulated signal has an ideal amplitude. However, an amplitude of a demodulated signal is not always an ideal one, but actually varies.
Hereinbelow is explained how an amplitude of a demodulated signal varies, with reference to FIG. 1.
A signal having been transmitted from a transceiver is first frequency-converted, and then, is converted into a voltage signal by means of a frequency-voltage (f-V) converter. A frequency-voltage conversion characteristic is basically linear. Herein, it is assumed that the conversion characteristic represented as "characteristic A " in FIG. 1 is an ideal one. However, an inclination of the conversion characteristic varies due to variance in characteristics of circuit parts constituting a f-V converter and temperature characteristic of a f-V converter. Hence, it is assumed that an inclination of the characteristic line is decreased, specifically, the characteristic A varies to the characteristic B.
As will be obvious in view of FIG. 1, if an inclination of the conversion characteristic of a f-V converter is decreased, an amplitude of a demodulated signal transmitted from a signal receiving circuit is decreased in proportion to a degree to which the inclination is decreased. On the other hand, if an inclination of the conversion characteristic of a f-V converter is increased, an amplitude of a demodulated signal transmitted from a signal receiving circuit is increased in proportion to a degree to which the inclination is decreased.
However, as mentioned earlier, the conventional quaternary FSK receiver carries out only to harmonize an average value of received data to the second reference. The first and third reference values are set equal to predetermined fixed values obtained on the assumption that a demodulated signal has an ideal amplitude. As a result, if an amplitude of a demodulated signal varies due to variation in characteristic of a signal receiving circuit, there may occur an error in judging a sign of a received signal.
Japanese Unexamined Patent Publication No. 7-154433 has suggested a quaternary FSK receiver comprising means for judging a sign of a signal by comparing detection signals in a quaternary FSK signal to reference levels, an average calculator for calculating an average of the detection signals, and a circuit for generating the reference levels by using the average as a reference value.
Japanese Unexamined Patent Publication No. 9-247219 has suggested an apparatus for judging a symbol indicated by a base band signal supplied from a demodulator.
Japanese Unexamined Patent Publication No. 10-145442 has suggested a radio signal selective calling receiver comprising first means for receiving a signal including a synchronization signal, and processing the thus received signal, second means for detecting a specific pattern in the synchronization signal, and storing the specific pattern as a specimen value, third means for determining optimal threshold values to be used as a reference value for judging a signal, based on the specimen value, and fourth means for judging a signal in accordance with an output of the third means.
However, the above-mentioned conventional quaternary FSK receiver and method are accompanied with the following problems.
First, since a reference value is changed only at a specific timing, if a firstly established reference value is a wrong one, it would take much time to correct it, and it would not be possible for the receiver to follow small fluctuation in a demodulated signal.
Second, there occurs an error in judging a sign of a signal, if an amplitude of a demodulated signal varies.